The coelacanth leads a monogamous life

September 19, 2013

Scientists have successfully analyzed the genetic make-up of the offspring of pregnant coelacanth females for the first time. They found that the likelihood that the offspring is fathered by one single individual is very high – unlike with many other fish species.

Dr Kathrin Lampert from the Ruhr-Universität Bochum and Prof Dr Manfred Schartl from the University of Würzburg, together with their colleagues, report about their findings in the journal Nature Communications.

Analysis of the microsatellite DNA

The pregnant coelacanthfemales studied by the researchers were about to give birth to their offspring. One female that had accidentally ended up in a trawl net by the Mozambique coast was carrying 26 embryos, another one caught unintentionally by fishermen in Zanzibar waters was carrying 23. When comparing 14 characteristic spots in the genetic make-up of the females and of their offspring with each other, the researchers found numerous overlaps. They deployed a method that is also utilised for conducting paternity tests in humans, namely microsatellite analysis. Microsatellites are short DNA sequences, consisting of only a few units that, typically, may recur up to 50 times. They do not usually carry any genetic information, but they are passed down from both parents. "As we know the mother's genotype, we are able to demonstrate by means of the microsatellite analysis that coelacanth offspring have one single father," says Manfred Schartl. Consequently, coelacanth females must be monogamous – at least for a certain period of time. The team also reconstructed the "hypothetical genotype", i.e. the hypothetical genetic make-up of both fathers.

Coelacanths do not take advantage of multiple mating

It is not clear why the females mate with one single male each. Mating with several males increases the chance of successful fertilisation, results in a higher genetic variability in the offspring and ensures that the best genes are passed on. It is possible that the advantages of multiple mating do not outweigh the costs for the female: increased energy input when searching for new males, danger of falling prey to predators, and an increased risk of infection.

No mating with relatives

The researchers discovered another interesting detail in the coelacanth's genetic make-up: father and mother of the offspring were not more closely related than the majority of random couples in a coelacanth population. This could mean that the females avoid mating with close relatives. Or that other features are more relevant in the choice for a suitable mate, for example size and frame or resistance against parasites.

Three years of pregnancy

In many fish species, fertilisation takes place outside the body. The females lay their eggs in a quiet spot in their aquatic environment; subsequently, the males – it can be several – add their semen. The offspring grow up in the water without their parents' protection – whereas the coelacanth gives birth to fully developed young. Scientists estimate that the "pregnancy" takes about three years.

Coelacanths were thought to be extinct

Until December 23, 1938, scientists had been convinced that coelacanths were extinct. Only a few fossilised prints gave evidence that those animals had existed more than 300 million years ago. Then, fishermen by the South-African coast discovered a greyish blue fish with a length of some 1.50 m and a weight of some 52 kg in their trawl net: the first specimen of a living coelacanth. Today, the existence of some 300 specimen has been proved. It is very difficult to get hold of tissue samples of a complete litter of a pregnant coelacanth female. This was made possible by the co-authors of "GEOMAR" in Kiel and Tutzing who have been researching the coelacanths' habits and occurrence for many years.

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The monogamy is probably for a more practical reason. How easy is it to FIND another member of your species in the dark? Especially if you're not a populous species to start with? Maybe it isn't a matter of what the partner looks like but rather whether they can find one at all.

Some squids solve the problem in a different way. The males inject sperm packets into the female anywhere they can reach. The squid only mate once, and are solitary, so it's a matter of mate at the first opportunity. The only disadvantage is that they aren't always choosy, and males have been found with other males' packets in them. In at least one case one was found with his OWN packet in him. OUCH...

Ovivivaparous - that's even more fascinating. Deep ocean, slow-living, monogamous and ovivivaparous. I'd say it was a situation based on slow reproduction, high parental investment, and little environmental variation. It would seem that they can afford to be choosy, and instead make sure to engage in disparate matings rather than catch-as-catch-can based on relatedness. That would, to me, argue against the can only find one in the dark idea - if they can be choosy, it could mean they either migrate upon birth to avoid related mates, or have a mechanism to identify mates with more disparate genetic makeup. Either one would argue for investment in variability being a behavior rather than a forced environmental circumstance.

The three year gestation... wow. That is a lot of parental investment.

Gmr: Possible. The population density would have to be high enough that they could find multiple choices, but I've never seen any report on that, so you may be right. And there are ways to find each other besides vision. After all, they find food!

No doubt their rarity makes them unwittingly monogamous. They mate with one male and he is the father of all the offspring. It is possible that once the female has mated, she is no longer interested in any more matings. The males no doubt sense the presence of a female through her pheromone trail, and that trail would present a different signature once the female has been impregnated because of the hormonal changes that she undergoes. That makes the monogamous nature of the coelacanth much less amazing than that of the Blue Marlin, who are not only monogamous, but who mate for life, considering their much greater number in clearer waters.

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